Roberts R66

[LyntonP]

The selenium HT rectifier in my Roberts R 66 battery mains portable has failed. I have read that this can be replaced with a 1N4006 diode and a 100 ohm resistor in order to give the correct HT half wave rectified DC voltage. Three questions.
What is the correct HT voltage?
Where exactly in the circuit should this be measured?
What wattage resistor should be used?
Thanks
Lynton

[ppppenguin]

I assume you have the circuit. It's Trader sheet 1273.

There are 2 rectifiers, a biphase one with 2 diodes for LT, a single half wave for HT. 1N4006 or 4007 with 100R is fine. Resistor value not critical.

Since there's a 3K9 resistor between the reservoir capacitor and the rest of the circuit that will drop the rectified HT significantly. I'd say nominal 90V (same as a B126 battery) after the resistor though not very critical as the battery voltage would vary over a fairly wide range as the battery ages.

If you haven't already done so, please replace the 10n capacitor (C23 on Trader sheet) that couples audio to the grid of the output valve. The original is likely to be leaky which could easily cause the audio output valve and transformer to suffer.

[LyntonP]

Thank you Jeffrey.
Lynton

[LyntonP]

I have ordered a replacement for C23 as advised, also a 47nf for C14 which is a paper cap, and also C25 a 20uf electrolytic which seems to be leaking.
Need some rexine for the case now!
Lynton
PS The HT now measures 97 volts measured across the plus to minus rails.

[ppppenguin]

You should replace ALL the paper caps. For example leakage in C14 will wreck the AGC, though leakage in C22, C16, C5 (screen decouplers) probably won't make much difference. The big LT smoothers (C29,30,31) all 2500uF may all be below par, resulting in hum. For neastest results the modern 2200uF replacements can be hidden in the old cans.

By chance I have an R66 which I'm taking to Harpenden on 3rd March to sell. It all looks untouched (I certainly haven't touched the innards), the case would come up nicely with some red shoe dye.

[LyntonP]

There was only one paper cap inside so I will change it as a matter of course plus the others as mentioned. I did try it for a very short time and it is now working fine. The outside of the case is rather tatty but I cannot complain as the radio was given to me.  It would be nice to recover it but I have been unable to locate any Rexine locally.  Shame as it used to be  manufactured in Manchester just down the road from me.
Your R66 is tempting but I also have a Grundig Yacht Boy on the bench at the moment which I would like to finish before moving on to anything else.
Once again thanks for all the help.
Lynton

[LyntonP]

    Very kindly presented with a Roberts R66 last weekend. Totally silent. Checked LT and it was ok at 2.6v, but HT was down to about 40v. Replaced selenium rectifier with a diode and resistor and after some experimentation with the resistor achieved 90v HT.
Set working well but on the advice of a Senior form member took the precaution to change the output grid and AGC line capacitor plus one electrolytic which seemed to be leaking. Thanks for the advice Jeffrey.
The case was rather dirty so spent an evening giving it a good scrub and polished it with saddle and harness cream to give it a nice sheen. Speaker grill backing fabric was rotten so this was replaced. Handle was missing so made a new one (not same colour but looks ok).
Set plays well and even picks up stations in my metal shed through the ferrite aerial.

[Crackle]

2.6v LT thats high Its surprising the valve heaters have not all gone. Where, how did you measure that voltage.
Mike

[LyntonP]

I measured it at the rectifier. What should the value be 1.5?
Lynton

[Crackle]

I would check it at the at C29 (trader) where it should be NO more than 1.5v DC, preferably 1.35v or something like that, whilst the radio is switched on, particularly if the radio is going to be used on mains a lot.

Mike